Radio frequency signals received by antennas have polarization properties that characterize the orientation of the incident electric fields. Communication systems antennas are generally designed to receive radio frequency signals having a specified polarization. However, in practice, incident fields matching the design polarization for a given antenna are not always received. If the polarization of the incident field approximates the polarization of the antenna, most of the signal power is received by the antenna, and the difference between the ideal and actual polarization is characterized by “polarization loss.”
When such communication systems require narrow beamwidth antennas for signal reception, the antennas must be accurately pointed at the received signal source to receive the maximum signal power. High gain antennas having a narrow beamwidth commonly use monopulse designs that can align the position of the antenna with the signal to receive the maximum available power; the deviation between ideal and actual antenna pointing is referred to as “pointing loss.”
Closed loop monopulse tracking systems operate by processing two types of antenna patterns. The first is a sum pattern that has its maximum gain when aligned with the signal and is used for data reception. The second is a difference pattern where a pattern null exists on the axis of the sum beam. The ratio of the difference and sum beams has a linear variation for deviations from the antenna axis, and this linear deviation is used to configure a control system that automatically maintains antenna alignment with the signal as the relative positions of the antenna and signal direction vary.
In operation, the antenna control system treats the difference beam output as an error signal. Minimizing this error signal aligns the null of the difference pattern with the signal direction. By design, the peak of the sum beam for the data reception is coincident with the null of the difference beam. Thus, the antenna alignment with the null of the difference beam also aligns the peak of the sum beam used for data reception. Because the control system continuously monitors the error signal, the antenna follows the movement in the signal direction. These sum and difference patterns are commonly obtained by two different antenna feed designs. One design surrounds the central feed horn for data reception with smaller tracking elements that are sampled for the difference beam. Another design is a multi-mode horn where the dominant mode provides the sum beam and higher order modes provide the difference beam.
While such antenna tracking techniques perform very well when the incident signals have the design polarization characteristics, performance degrades when the incident polarization is cross-polarized with the design polarization. By way of example, for telemetry antennas on spacecraft, cross-polarized signals can result from antenna interactions with the surrounding spacecraft. Also by way of example, for ground tracking stations supporting launch operations, cross polarization can result from interactions with the launch vehicle and plume effects.
The cross polarized signal can result in unstable tracking performance and the antenna failing to track the desired signal. The reason for this loss of tracking lies with the cross-polarized antenna response.
When the antenna receives a cross polarized signal, the difference over sum ratio of the tracking output of the antenna no longer has the desired linear variation as the antenna position deviates from the antenna beam axis. Thus, the control system for antenna pointing does not have the assumed linear variation with antenna displacements from the axis. Instead of the desired linear deviation of the error signal as the antenna moves from its desired axial position, the cross polarized response has a peak value on axis, the “zero error condition” is double valued at positions displaced from the axis, and the slope of the response is opposite the desired slope driving the antenna away from axis.
FIG. 6 shows a difference pattern for an example principal polarization response. As shown, the zero error condition occurs on-axis with positive and negative excursions as the antenna deviated from axis. Measured data also presents a well-behaved angular step response; likewise the angular ramp response which scans the antenna back and forth about the axis follows the linear deviation of the difference pattern.
FIG. 7 shows a difference pattern for an example cross polarization response. In contrast with the difference pattern of FIG. 6, the zero error point is not on axis but rather off-axis at a level about 10 dB down from the beam peak. Worse yet, the slope of the difference pattern on the left side is opposite that of the principal polarization and instead of driving the antenna toward the axis drives the antenna away from the axis. This behavior has also been observed in measured data. The step response indicates the unstable behavior of the antenna. The ramp response, unlike that of the principal polarization, does not have any indication of the linear variation on which the control system is predicated. The antenna response during the measurements is basically a coning motion corresponding to the zero error value being at the 10 dB below beam peak level and the instability is exhibited in both the azimuth and elevation channels. In operation, as the antenna continues to search, it encounters the wrong slope and drives the antenna away from the signal. Thus, the antenna response is unstable when tracking the cross polarized signal.
For the data channel, diversity-combining techniques exist to maximize the received signal level. And for the tracking, two orthogonally polarized signals can be used with independent tracking receivers. Such an approach, however, requires the selection of the appropriate signal polarization and since either orthogonal polarization is not matched to the incident signal, loss of signal power occurs.
In summary, tracking performance is degraded by polarization mismatch loss, and antenna tracking can become very unstable when the received signal is predominately cross-polarized. Conventional antenna control systems expect an output from the tracking (difference) beam that linearly increases in amplitude with deviations from the boresight axis of the antenna. This linear increase in amplitude with deviation from the boresight axis is the pattern behavior in the null of the difference beam. Cross-polarized signals do not have this expected linear response resulting in unstable tracking behavior.
Two problems therefore are faced when the received signals have mixed and time-varying polarization properties. The first problem is the signal reduction that results from polarization mismatch, which in turn, results in a reduction in system sensitivity. The second problem is the degradation in antenna tracking performance and possible loss of antenna tracking resulting from the unstable behavior. It would be desirable to be able to provide a method that avoids or lessens this unstable operation and results in antenna tracking that is insensitive to the polarization of received signals.